The present invention relates to a room with low reflections in which sound waves are strongly absorbed within the entire auditory range of 20 Hz to 16 KHz such that (in particular, in small rooms) sound fields are created only by the sources in the room without disturbing reflections from the walls, the ceiling and (under certain circumstances) the floor.
Waves emitting from sound sources in a free sound field (reflection-free surfaces) form a typical acoustic field. The human ear, also manmade acoustic sensors, with which the sound field (e.g., as a musical performance) is subjectively perceived or (e.g., as emission from a noise source) objectively judged, react very sensitively to sound wave reflections from the surrounding boundaries in a room. Therefore, enclosed rooms need to be covered with more or less acoustically absorbing peripheral surfaces which absorb the impinging sound waves strongly enough so that the reflections from the surrounding surfaces (generated by one or multiple sound sources) do not disturb the freefield condition in the room. This falsification must be neither subjectively perceivably, nor objectively measurably beyond certain limits within the entire audible range of interest from approximately 20 Hz to approximately 16 KHz.
State-of-the-art anechoic room coverings fulfill their function with a conventional covering depth of up to approximately 1 m, from approximately 80 Hz upward. In order to dampen 20 Hz reflections effectively, walls, ceiling and floors of rooms would have to be covered on all surfaces to a depth of a few meters with conventional sound absorbers (usually mineral wool). As all-surface room coverings of this type require a lot of space, as well as complicated mounting, anechoic rooms intended for measuring purposes are usually designed only for frequencies above 100 Hz with a covering of barely 1 m in depth. In rooms intended for hearing or measuring only low frequencies, according to the state of the art, less covering depths and additional "edge absorbers" with correspondingly greater depths are built into the corners and edges of the rooms as special "low frequency absorbers" (Everest, F. A.: The Master Handbook of Acoustics: New York; McGraw-Hill 1994, pp. 342 ff.).
When designing high-grade sound studios, acoustic specialists try to create, in general, a certain "room impression", in particular, in which to play music. The widespread attempts to obtain a certain "diffusivity" in the room, via reflections, inevitably leads (at low frequencies) to excitation of the cavity resonances of the room and consequently to falsifying the sound occurrences. This manifests itself as an unpleasant "droning" in the room.
The currently valid standards and guidelines (DIN 45 635, Part 1: Gerauschmessung an Maschinen. Erlauterungen zu den Gerauschemissions-Kenngrossen. ISO 37 45: Acoustics, determination of sound power levels of noise sources, Precision methods for anechoic and semi-anechoic rooms) set forth strict criteria:
(a) According to the standards and guidelines, the degree of absorption of the room cladding should be at least 99% (at vertical sound incidence). In order for the reflections of the peripheral surfaces of the room to remain under 1% of incident sound energy, the prevailing opinion is to make extreme demands on material and design for the acoustical room covering which cannot be met by a normal, sound absorbing layer in front of the peripheral surfaces. PA1 (b) To the drafters of the pertinent standards, realization of the extreme criteria for anechoic rooms (as stated in (a) above) seems only possible using covering depths of one quarter of the wavelengths of the deepest frequency to be measured (e.g. 1 m for 80 Hz and 2.50 m for 30 Hz). PA1 (c) Someone skilled in the art knows that such thick absorbing layers cannot be realized with a sufficiently small flow resistance in order to even come close to meeting the extreme criteria (as discussed in (a) above). However, the pertinent standards suggest a very uneven covering of wedges, pyramids or cubes made of special fibrous or porous damping material. The prevailing opinion is that if the sound waves impinge vertically into these structures, they can be ensured sufficient depth of penetration and thus almost full absorption. PA1 (d) Finally the valid abovementioned standards for designing anechoic rooms for precision measuring uniformly prescribe that the absorbing covering should be distributed in the same manner and evenly over all the peripheral surfaces. This prescription, in particular, suggests that the special problems of the free sound field condition for low frequencies in small rooms apparently has not been properly understood (Zha, X.; Fuchs. H. V.; Spah, M.: Messung des effectiven Absorptiongrades in kleinen Raumen. Rundfunktechn. Mitteilung 40 (1996), H. 3, S. 77-83).
Moreover, these same guidelines and textbooks prescribe that measurement points in a room should always maintain a distance of a quarter of the wavelength (.lambda.) from the covering (for instance from the tips of the wedges) and a distance (.lambda.) from the sound source. This yields, depending on the size of the assumedly cubic source (located in the center of a cubic all surface conventional anechoically clad room), the bottom critical frequency which is dependent on the unclad-construction volume shown in FIG. 1. According to this, in order to still be able to measure at, e.g., 50 Hz, the room would have to be made 10000 m.sup.3, which due to cost and space limitation is impossible in practice. In rooms, which are usually smaller than 700 m.sup.3, according to these widespread conceptions (even at small sound sources), only measurements above approximately 125 Hz can be conducted. With these design criteria, about 30% of the unfinished construction volume must still be wasted on the thick covering of the room! If the room is not cubic or cuboid in shape and/or the source is moved out of the center of the room, measuring low frequencies is even more difficult.
As a result of various prejudices (regarding sound studios and rooms for precision measuring, in particular), small rooms with a volume of 50 to 400 m.sup.3 have subjective and objective drawbacks at low frequencies below 125 HZ. Consequently, voluminous "bass traps" are scattered about in sound studios (FIG. 2(a) thru 2(d)). This impairs the sound field of the room. Precision measuring rooms are clad (e.g. according to FIG. 3) with wedges up to 3 m in length.
Thus an object of the present invention is to provide a low reverberating room that is able to absorb 95% of sound from 16 KHz down to 25 Hz.
This and other objects and advantages are achieved by the anechoic room according to the present invention in which the walls and ceilings are covered with sound absorbing materials.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.